Type 4 phosphodiesterases (PDE4s) are metallohydrolases that catalyze the hydrolysis of cAMP to AMP. At the bottom of its active site lie two divalent metal ions in a binuclear motif which are involved in both cAMP binding and catalysis [(2000) Science 288, 1822−1825; (2000) Biochemistry 39, 6449−6458]. Using a SPA-based equilibrium [³H]rolipram binding assay, we have determined that Mg²⁺, Mn²⁺, and Co²⁺ all mediated a high-affinity (K_d between 3 and 8 nM) and near stoichiometric (R)-rolipram binding to PDE4. In their absence, (R)-rolipram binds stoichiometrically to the metal ion-free apoenzyme with a K_d of ∼150 nM. The divalent cation dose responses in mediating the high-affinity rolipram/PDE4 interaction mirror their efficacy in catalysis, suggesting that both metal ions of the holoenzyme are involved in mediating the high-affinity (R)-rolipram/PDE4 interaction. The specific rolipram binding to the apo- and holoenzyme is differentially displaced by cAMP, AMP, and other inhibitors, providing a robust tool to dissect the components of metal ion-dependent and independent PDE4/ligand interactions. cAMP binds to the holoenzyme with a K_s of 1.9 μM and nonproductively to the apoenzyme with a K_d of 179 μM. In comparison, AMP binds to the holo- and apoenzyme with K_d values of 7 and 11 mM, respectively. The diminished Mg²⁺-dependent component of AMP binding to PDE4 suggests that most of the Mg²⁺/phosphate interaction in the cAMP/PDE4 complex is disrupted upon the hydrolysis of the cyclic phosphoester bond, leading to the rapid release of AMP.